1. Field of the Invention.
This invention relates to determination of electrolytes, and more particularly relates to a composite membrane and to an electrolyte sensing device including the membrane useful in making the determination.
2. Background of the Invention
Hydrophilic polymeric membranes are of interest to the industry for numerous applications ranging from extracorporeal blood purification (hemodialysis) to analysis of blood gases and electrolytes. For a typical biomedical application, a suitable hydrophilic membrane should have high ion/solute. permeability, mechanical strength and blood compatibility. Another desirable property, particularly in an analytical medical device application, is dimensional stability of the membrane upon absorption of water.
The ionic permeability of most membranes known in the art depends on their water absorption, which is generally accompanied by swelling. Any significant swelling in turn leads to dimensional changes. These changes represent a significant drawback in applications of the membrane in biosensing devices contemplated for contact with body fluids.
Polypropylene (PP) is a hydrophobic material, nonwettable by water, and thus, even when porous, it is not permeable to ions in an aqueous solution unless a positive pressure gradient is applied. It can be converted to a hydrophilic membrane with excellent dimensional stability and mechanical properties by treatment with surfactants (i.e. Celgard.sup.R 3500, Celanese). The surfactant-treated porous PP, however, is unsuitable for blood and tissue contact applications because the surfactant leaches out of the membrane matrix causing cell lysis.
It is known in the art that plasma surface treatment techniques can be utilized to change the surface energy of polymeric films. A coating of plasma-polymerized ultra thin film changes permanently the surface energy of a substrate without altering bulk properties of the material. Composite membranes based on plasma induced polymerization-deposition of polymer directly onto porous and nonporous substrates have been disclosed by Zdrahala et al. (Abstracts, The 1987 International Congress on Membranes and Membrane Processes, Tokyo, Japan, June 1987. page 477) wherein dimensionally stable hydrophilic membranes are prepared by plasma polymerizing gaseous acrylic acid with concomitant deposition of a layer of polyacrylic acid (PAA) on the PP. These membranes, however, showed only marginal improvement in hydrogen ion diffusion through the disclosed membrane.
Lazear, in GB patent application 2,058,802A discloses a polyolefinic open-celled microporous film rendered hydrophilic by chemically affixing PAA to the pore surfaces. The porous films are limited to those having interconnected pores, commonly referred to as depth filters, and are prepared by coating the pores with acrylic acid and polymerizing with ionizing radiation.
In U.S. Pat. No. 4,717,479, Itoh et al. discloses a porous hydrophilic polyolefin membrane consisting of a hydrophobic polyolefin membrane having surface-grafted thereto a polymerized surface active monomer. The membrane may be of any type, including hollow fiber, planar and tubular types, and the monomer includes a polymerizable olefin group, a hydrophobic group and a hydrophilic group. A method for preparing the membrane includes applying the monomer to the membrane and polymerizing by application of heat or radiation in the presence of a polymerization catalyst.
Sensing devices for determination of blood components are well-known. All such devices utilize a membrane which is permeable to the blood component being analyzed. U.S. Pat. Nos. 4,534,356 and 4.536.274, to Papadakis disclose electrochemical sensors in which membranes useful for blood gas analysis are broadly defined as hydroqels or hydrophilic polymers or copolymers and membranes useful for blood pH determinations are copolymers of fluorine-containing monomers.
A portable assembly for analysis of blood oxygen and carbon dioxide which includes a blood sampler, an electrochemical sensor and blood gas analyzer is disclosed by Cronenberg et al. in U.S. Pat. No. 4,615,340. The sensor includes a gas permeable, ion permeable membrane fabricated of polycarbonate or cellulose and a gas permeable, ion impermeable membrane of polytetrafluoroethylene or PP.
Boold gases are measured by Lubbers et al. in U.S. Pat. No. 31,879 by a fluorescence-based sensor using selective qas permeable membranes and optical fibers to direct incident light to a dye and fluorescence from the dye.
A fiber optic pH probe for physiological studies using an ion permeable cellulose membrane is described by Peterson et al. in U.S. Pat. No. 4,200,110.
Baxter, in U.S. Pat. No. 4,505,799, discloses an ion sensitive field effect transistor (ISFET) for measurement of hydrogen ions which includes a membrane which may be silicon nitride or aluminum oxide.
There is a need for a membrane which combines the high ion permeability of hydrogels with the dimensional stability and meschanical strength of polyolefins. The present invention satisfies this need.